1. Field of the Invention
The present invention relates to a magnetic read/write system uses a fixed MR head to read magnetically recorded data from a magnetic recording medium, and to such a magnetic recording medium. More particularly, the present invention relates to a magnetic read/write system that employs a fixed MR head to read magnetically recorded data from a magnetic recording medium, with improved running durability, adherence to a head, and wear resistance and to such a magnetic recording medium
2. Description of the Related Art
As the recording density of magnetic recording media increases, friction between a magnetic layer and components of a reading device during running has posed a significant problem and there is an increasing need to improve running durability, adherence to a head, and wear resistance of such recording media.
To meet such needs, techniques have been developed where a fatty acid ester is added to a magnetic layer of a magnetic recording medium that is mainly composed of ferromagnetic powder and binder resin (Japanese Patent Laid-Open Publication No. Sho 50-22603, Japanese Patent Laid-Open Publication No. Sho 50-153905, Japanese Patent Laid-Open Publication No. Sho 53-149302, Japanese Patent Laid-Open Publication No. Sho 55-139637, Japanese Patent Publication No. Sho 39-28367, Japanese Patent Publication No. Sho 41-18065, and Japanese Patent Publication No. Sho 47-12950).
These techniques, however, suffer a problem that many of the ester compounds that have straight-chain alkyls, which bring about their relatively high lubricating performance, have high melting points and thus form deposits on the surface of the magnetic layer at low temperatures. To cope with this, some techniques employ a fatty acid ester having a saturated or unsaturated branched hydrocarbon group with high molecular weight as an additive to the magnetic layer (Japanese Patent Publication No. Sho 47-12950, Japanese Patent Laid-Open Publication No. Sho 58-218038, Japanese Patent Laid-Open Publication No. Sho 60-205827, Japanese Patent Laid-Open Publication No. Sho 61-294637, and Japanese Patent Laid-Open Publication No. Sho 62-125529). One technique involves adding to the magnetic layer a fatty acid ester having the following general formula: 
where R11 is a straight-chain saturated alkyl having 6 to 12 carbons, R12 is a straight-chain saturated alkyl having 4 to 10 carbons, and R is a straight-chain or branched alkyl having 4 to 22 carbons (Japanese Patent No. 2559259).
One type of magnetic read/write system, which has recently been put to practical use and is intended for use in a computer back-up system, reads data stored in a magnetic recording medium using a fixed MR head that operates at a relative speed of 2.0 to 5.0 m/s with respect to the recording medium. The system, known as the linear tape drive system, operates on the basis of linear scanning, in which a magnetic tape (which may be referred to simply as a tape, hereinafter) is moved along its length with respect to the fixed MR head as the data is read along that direction. Unlike a helical scanning system, which employs a rotary head, the magnetic tape is moved at high speeds in this system. As a result, the tape is vigorously rubbed against the head or guide rolls, causing the magnetic coating to come off the tape. For this reason, the role of the lubricant has become ever more significant to minimize damage to the coating, and should the coating come off the tape, it is desirable the coating does not stick to the surrounding area of the head gap.
No conventional fatty acid ester has ever achieved satisfactory performance when added to the magnetic layer in such a system. For example, fatty acid esters that have a saturated or unsaturated branched hydrocarbon with high molecular weight are less than satisfactory in terms of film strength and lubricating performance of the magnetic layer under low temperature conditions.
Fatty acid esters used in a magnetic disk as described in Japanese Patent No. 2559259 are less likely to crystallize and remain in a liquid state at relatively low temperatures, exhibiting lubricating property. These fatty acid esters do not readily evaporate from the surface of the magnetic layer at high temperatures, nor do they form deposits at low temperatures since fatty acid units and alcohol units present in the molecule have a limited number of carbons. Thus, the fatty acid esters, are capable of providing high running durability under various conditions. However, the lubricating performance of these fatty acid esters is not sufficient because of their high dynamic viscosity. Therefore, a high friction results when the fatty acid esters are applied to the tape. This causes abrasion of the coating.
The present invention addresses the above-identified problems associated with the prior art.
Accordingly, it is an objective of the present invention to provide a magnetic read/write system that uses a fixed MR head to read magnetically recorded data from a magnetic recording medium that has improved adherence to a head, running durability and wear resistance under low temperature conditions, as well as at room temperature, and to provide such a magnetic recording medium. In particular, the present invention aims at provision of a magnetic read/write system that uses a magnetic tape exhibiting improved performances in terms of the above-described adherence, running durability, and wear resistance when used in a linear tape drive system, in which the fixed MR head reads magnetically recorded data from a magnetic recording medium formed as a tape (i.e., a magnetic tape), such as DLT4, while operating at a relative speed of 2.0 to 5.0 m/s with respect to the recording medium. It is also an objective of the present invention to provide such a magnetic tape.
In the course of their studies to find a solution to the above-described problems, the present inventors have found that the above-described objectives can be achieved by providing a non-magnetic layer that contains a specific fatty acid ester and a fatty acid between a non-magnetic support and the above-described magnetic layer and ultimately completed the present invention.
In one aspect, the present invention provides a magnetic read/write system, in which a fixed MR head serves to read magnetically recorded data from a magnetic recording medium as it operates at a relative speed of 2.0 to 5.0 m/s with respect to the magnetic recording medium comprising a non-magnetic support and a magnetic layer, wherein a fatty acid ester represented by general formula (I): 
where R1 is a hydrocarbon having 4 or less carbons, and R2 is a straight-chain hydrocarbon having 12 or more carbons, exists between a read element of the MR head and the magnetic layer.
In another aspect, the present invention provides such a magnetic recording medium comprising:
a non-magnetic support;
a magnetic layer containing a ferromagnetic powder and a binder resin, the magnetic layer formed over the non-magnetic support and having a dry thickness of 0.5 xcexcm; and
a non-magnetic layer containing a non-magnetic powder and a binder resin, the non-magnetic layer interposed between the non-magnetic support and the magnetic layer; the non-magnetic layer containing as a lubricant a fatty acid ester represented by general formula (I): 
where R1 is a hydrocarbon having 4 or less carbons, and R2 is a straight-chain hydrocarbon having 12 or more carbons, and a fatty acid having 12 or more carbons.
The fatty acid ester of the general formula (I) for use with the present invention, which is derived from fatty acids having a hydrocarbon branch at 2xe2x80x2-position, has a low solidification point and does not suffer a significant decrease in the lubricity in low temperature conditions. In addition, this fatty acid ester has a shorter fatty acid side chain as compared to the conventional fatty acid esters as represented by the general formula (II) and thus exhibits low dynamic viscosity and shear viscosity even in low temperature conditions. Accordingly, by using the fatty acid ester, not only can the friction and the wear in the tape be reduced in a wide temperature range, but also the durability under various environments is improved. Furthermore, in terms of physical properties, the fatty acid ester exhibits less oiliness, the property being characteristic of fatty acid esters. This is believed to contribute to the improvements in the adherence to a head and stickiness of the tape. In the present invention, the fatty acid ester of the general formula (I) is added only to the non-magnetic layer and is allowed to gradually migrate from the non-magnetic layer through the magnetic layer to the surface thereof. In this manner, high durability and wear-resistance can be achieved even when the tape is moved at a relative speed of 2.0 to 5.0 m/s with respect to the fixed MR head. These effects are not obtained if the fatty acid ester is added only to the magnetic layer. The concurrent presence of the fatty acid with the non-magnetic powder in the non-magnetic layer makes it possible for the fatty acid ester to migrate in the manner described above.
The present invention will now be described in detail with reference to preferred embodiments.
A non-magnetic support for use in a recording medium of the present invention is a sheet of film and is properly selected from conventional resin films made of resin materials including, but not limited to, polyesters, polyamides, or aromatic polyamides, or resin films formed by laminating these films. Such films, as well as its thickness and other parameters, are known, and should not be limited to a particular one.
Ferromagnetic powder contained in the magnetic layer of the magnetic recording medium of the present invention is a ferromagnetic metal powder that has a needle-like shape and preferably has an average major axis length of 0.15 xcexcm or less, more preferably from 0.05 to 0.10 xcexcm. If the powder has an average major axis length exceeding 0.15 xcexcm the electromagnetic conversion characteristics (in particular, S/N and C/N characteristics) required of magnetic recording media will become insufficient.
Preferably, the ferromagnetic powder is contained in the magnetic layer composition in an amount of 70 to 90% by weight. If the amount of the ferromagnetic powder is excessively large, the amount of binder is reduced and, as a result, the surface smoothness of the recording medium after calendering worsens. Conversely, if the amount of the ferromagnetic material is excessively small, high read output cannot be achieved.
Examples of the binder resin for use in the magnetic layer include, but are not limited to, conventional thermoplastic resins, thermosetting resins, radiation cure resins, and mixtures thereof.
Preferably, the binder resin is contained in the magnetic layer in an amount of 5 to 40 parts by weight, particularly 10 to 30 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. If the amount of the binder resin is too small, the strength of the magnetic layer is reduced and running durability may be lowered. On the other hand, if the amount is too large, the amount of the ferromagnetic metal powder is reduced, resulting in a reduced electromagnetic conversion characteristic.
When a thermosetting resin is used as the binder resin, various known polyisocyanates can be used to serve as a cross-linking agent for hardening the binder resin. The amount of the cross-linking agent contained in the magnetic layer is preferably from 10 to 30 parts by weight with respect to 100 weight parts of the binder resin. If necessary, abrasives, dispersing agents such as surface active agents, higher fatty acids and various other additives may be added to the magnetic layer.
A coating for forming the magnetic layer is prepared by adding an organic solvent to the above-described components. The organic solvent may be one or more solvents properly selected from ketone solvents including methylethylketone (MEK), methylisobutylketone and cyclohexanone, and aromatic compound solvents including toluene. The use of other organic solvents is also contemplated. The amount of the organic solvent to be added is preferably from about 100 to about 900 parts by weight with respect to 100 parts by weight of the total amount of solid content (such as the ferromagnetic metal powder and various inorganic particles) and the binder resin.
In the present invention, the magnetic layer has a thickness of 0.50 xcexcm or less, preferably from 0.05 to 0.50 xcexcm, and more preferably from 0.10 to 0.25 xcexcm. The magnetic layer that is too thick may increase the self-magnetization loss and thickness loss.
The magnetic recording medium of the present invention includes a non-magnetic layer interposed between the above-described magnetic layer and the non-magnetic support. In this manner, the electromagnetic conversion characteristic of the thin-layered magnetic layer is enhanced and reliability of the recording medium is further improved.
One characteristic of the present invention resides in that the non-magnetic layer contains as a lubricant a fatty acid ester represented by the following general formula: 
where R1 is a hydrocarbon having 4 or less carbons, and R2 is a straight-chain hydrocarbon having 12 or more carbons. Among such fatty acid esters, cetyl 2-ethylhexanoate, stearyl 2-ethylhexanoate, myristyl 2-ethylhexanoate and stearyl 2-ethylbutanoate are preferred, cetyl 2-ethylhexanoate being particularly preferred.
By adding these fatty acid esters to the non-magnetic layer, the DLT4 durability as well as adherence to head can further be improved as compared to using conventional fatty acid esters. Also, the decrease in the durability due to discharge, which is seen when fatty acid esters with melting points of 30xc2x0 C. or higher are used, does not take place in relatively low temperature environments. Also, the fatty acid esters of the present invention, serving as a lubricant, have a smaller dynamic viscosity and a shear viscosity due to their relatively shorter carbon chains than those of the conventional fatty acid esters. Accordingly, damage to the coating caused by the friction between the magnetic layer of the magnetic tape and a back-coat layer is reduced. If the fatty acid ester having the general formula (I) has R1 containing 5 or more carbons or has its ethyl replaced with an alkyl with 3 or more carbons, the fatty acid ester becomes oily, which affects the adherence to a head and stickiness of the tape. If R3 has less than 12 carbons, the fatty acid ester tends to crystallize in low temperature conditions and tends to evaporate from the surface of the magnetic layer in high temperature conditions.
Preferably, the lubricant is blended in the non-magnetic layer in an amount of 0.2 to 5 parts by weight with respect to 100 weight parts of the non-magnetic powder. If the amount is less than 0.2 parts by weight, the effects cannot be achieved. If the amount exceeds 5 parts by weight improvements in the running durability or wear resistance of the tape are not as significant as expected from the amount. In addition, if the amount is excessive, significant problems arise especially with tapes, such as reduced durability due to discharge and the magnetic layer sticking to the back-coat layer.
The non-magnetic layer contains at least non-magnetic powder, binder resin, and fatty acid with 12 or more carbons and has a thickness of preferably 2.5 xcexcm or less, more preferably from 0.1 to 2.3 xcexcm. The thickness larger than 2.5 xcexcm does not improve performance of the non-magnetic layer any further. On the contrary, too large a thickness can often result in non-uniformity in thickness in a coating layer. Not only does this require stricter coating conditions but also may result in a reduced surface smoothness.
Various inorganic powders can be used as the non-magnetic powder for use in the non-magnetic layer. For example, needle-shaped non-magnetic powders, such as needle-shaped non-magnetic iron oxide (xcex1-Fe2O3), are preferably used. Various other non-magnetic powders, including calcium carbonate (CaCO3), titanium oxide (TiO2), barium sulfate (BaSO4), and xcex1-alumina (xcex1-Al2O3), may preferably be blended. The non-magnetic layer preferably contains a carbon black, example of which includes furnace black for rubber, thermal black for rubber; black for color and acetylene black.
The carbon black and the inorganic powder are preferably blended at a ratio of 100:0 to 10:90 by weight. The proportion of the inorganic powder greater than 90 may lead to a problem in terms of surface electrical resistance.
As with the case of the magnetic layer, the binder resin may be a conventional thermoplastic resin, thermosetting resin, radiation cure resin and a mixture thereof, with the radiation cure resin being particularly preferred.
It is essential that the non-magnetic layer of the present invention further contain as an additive a fatty acid with 12 or more carbons, preferably stearic acid. When necessary, it may further contain a dispersing agent, such as a surface-active agent, and various other additives.
A back-coat layer is optionally provided for the purposes of improving running stability and prevent static electricity from building up in the magnetic layer. The back-coat layer preferably contains from 30 to 80% by weight of carbon black, which may be any of commonly used carbon blacks and may be the same as that used in the non-magnetic layer. In addition to the carbon black, the back-coat layer may optionally contain non-magnetic inorganic powders, such as the abrasives used in the magnetic layer, a dispersing agent such as a surface-active agent, a lubricant such as higher fatty acid, fatty acid ester and silicone oil, and various other additives.
The back-coat layer has a thickness of 0.1 to 1.0 xcexcm, preferably 0.2 to 0.8 xcexcm (after calendering). The thickness greater than 1.0 xcexcm results in excessive friction between the recording medium and the path that the recording medium follows and is rubbed against. This leads to reduced running stability. On the other hand, the thickness less than 0.1 xcexcm results in the back-coat layer being abraded as the recording medium is moved.
The above-described recording medium of the present invention has improved adherence to a head, running durability, and wear resistance of the tape and thus is suitable for use with fixed MR heads under low temperature conditions as well as at room temperature. In an MR head, resistance of a read sensor, which uses a magnetic material, changes when the read sensor is exposed to magnetic field, which allows the head to read external magnetic signals. Output of an MR head is not affected by the relative speed of the head with respect to the recording medium, and for this reason, MR heads achieve high output when used to read magnetically recorded data recorded with a high track recording density. In order to achieve high resolution and high RF characteristic, a typical MR head has a construction in which a magnetoresistive film (MR film) is interposed between a pair of magnetic shield film (Shielded MR head).
In a preferred magnetic read/write system, data is first recorded magnetically on the magnetic recording medium of the present invention and the data is then read using a fixed MR head operated at a relative speed of 2.0 to 5.0 m/s. The fatty acid ester of general formula (I) added to the non-magnetic layer of the magnetic recording medium is allowed to exist between the read element of the MR head and the magnetic layer.